Mass spectrometry (MS) is used to measure the mass of a sample molecule, as well as the masses of fragments of the molecule, in order to identify that sample. Some mass spectrometers begin with a gaseous, electrically neutral sample that is ionized using an electron beam. The resulting ions are accelerated by an electric field to a magnetic sector where they are deflected to a detector. The mass to charge ratio (m/z ratio) of these ions can be calculated as a function of the path through, and the strength of, the magnetic field used to deflect the sample ions to the detector. The mass is typically expressed in terms of atomic mass units, also referred to as Daltons. The electric charge of the sample ions is typically expressed in terms of multiples of elementary charge. Sample ions can be singly or multiply charged, and for sample ions having only a single charge, the m/z ratio is the mass of the ion.
Another method for measuring the mass of sample ions is time-of-flight (TOF) mass spectrometry, which involves measuring how long it takes the sample ions, or fragments of them, to travel a known distance. Post Source Decay (PSD) studies of the charged fragments can be used to better understand the structure of the sample. After fragmentation, the resulting pieces all travel at the same speed and arrive at the detector at the same time, where they are repelled by an electric field. The reflected ions have different speeds that depend on their individual masses, which can be measured to better understand the molecular structure of the corresponding sample.
Molecules that are not easily rendered gaseous can be difficult to study using MS. Accordingly, advances in MS often address problems associated with handling liquid or solid samples.
Desorption MS is a mass spectrometric technique useful with solid and liquid samples. Sample molecules are first adsorbed on a substrate. Later, they are desorbed (i.e. removed) from the substrate. Desorption MS became even more useful, particularly for the study of biomolecules such as proteins, by a technique that has come to be known as “matrix-assisted laser desorption/ionization” (MALDI). In contrast to MS techniques that use an electron beam to ionize a sample, MALDI uses an organic matrix to transfer a proton to the sample during sample vaporization. In MALDI, the sample is typically dissolved in a solid, ultraviolet-absorbing, crystalline organic acid matrix. The matrix vaporizes upon pulsed laser irradiation; the sample is carried along with the vaporized matrix to the detector.
In MALDI, salts and buffers can interfere with the formation of the matrix crystal and result in loss of signal. MALDI is also severely limited in the study of small molecules because the MALDI matrix interferes with measurements of ions having an m/z ratio below about 700. This mass region is sometimes called the low-mass region and varies somewhat depending on the matrix used. Although MALDI-MS (matrix-assisted laser desorption/ionization mass spectrometry) analysis can be utilized for small molecules, and matrix suppression can be achieved under certain circumstances, matrix interference presents a real limitation of the study of the low-mass region via MALDI-MS.
Even with large molecules, MALDI has significant limitations. The matrix and matrix fragments can form adducts with the sample ion. Adduct formation results in a broadening of the sample signal over a range of molecular weights.
MALDI is also limited in studying the Post Source Decay (PSD) of molecules because the vaporized matrix molecules of the sample interfere with the measurement of the fragments after reflection, rendering MALDI impractical even for molecules with a molecular weight over 700 Daltons.
Direct desorption/ionization without a matrix has been studied on a variety of substrates. Some of the more recently developed substrates are described in the following publications and patents: “Desorption-lonization Mass Spectrometry on Porous Silicon” by Jing Wei, Jillian M. Buriak and Gary Siuzdak, Nature, vol. 399, pp. 243–246 (1999); PCT Application Publication Number WO/00/54309 to Gary E. Siuzdak, Jillian Buriak, and Jing Wei entitled “Improved Desorption/lonization of Analytes from Porous Light-Absorbing Semiconductor,” which was published on Sep. 14, 2000; U.S. Pat. No. 6,288,390 to Gary E. Siuzdak et al. entitled “Desorption/lonization of Analytes from Porous Light- Absorbing Semiconductor,” issued on Sep. 11, 2001; “Desorption-lonization Mass Spectrometry Using Deposited Nanostructured Silicon Films,” by Joseph D. Cuiffi, Daniel J. Hayes, Stephen J. Fonash, Kwanza N. Brown, and Arthur D. Jones, Anal. Chem, vol. 73 (2001) pp. 1292–1295; U.S. Patent Application Publication 2002/0048531, published on Apr. 25, 2002; U.S. Pat. No. 6,399,177 to Steven J. Fonash, Ali Kaan Kalkan, and Sanghoon Bae entitled “Deposited Thin Film Void-Column Network Materials, which issued Jun. 4, 2002; U.S. Patent Application Publication 2002/0187312 entitled “Matrix-Free Desorption Ionization Mass Spectrometry Using Tailored Morphology Layer Devices” by Steven J. Fonash, Ali Kaan Kalkan, Joseph Cuiffi, and Daniel J. Hayes, which was published on Dec. 12, 2002; “Desorption/lonization on Silicon (DIOS) Mass Spectrometry: Background and Applications” by Warren G. Lewis, Zhouxin Shen, M. G. Finn, and Gary Siuzdak, Int. J. Mass. Spectrometry, vol. 226, (2003), pp. 107–116; U.S. Patent Application Publication 2003/0057106 entitled “High Throughput Chemical Analysis By Improved Desorption/lonization on Silicon Mass Spectrometry” by Zhouxin Shen and Gary Siuzdak, which was published on Mar. 27, 2003; U.S. Pat. No. 6,707,036 and U.S. Pat. No. 6,707,040, both to Alexander A. Makarov and Pavel V. Bondarenko, both entitled “Ionization Apparatus and Method for Mass Spectrometer System,” both of which issued on Mar. 16, 2004, all hereby incorporated by reference. Perhaps the best substrate to date, which is described in at least some of the aforementioned patents and publications, is porous silicon (the acronym DIOS refers to “Desorption/lonization on Silicon”). The '390 patent, for example, describes the use of porous, light absorbing semiconducting substrates for the desorption/ionization of analyte. These substrates were used as replacements for the organic matrix in MALDI for analyzing proteins and other biomolecules by laser desorption/ionization mass spectrometry. Illumination of the semiconductor results in desorption/ionization of analyte, and the ionized analyte can then be detected. An advantage of using the porous silicon substrate relates to the ability to perform measurements without an organic matrix, making DIOS more amenable to small molecule analysis. The absence of a matrix completely avoids the types of interference that result when a matrix is used. Also, DIOS can be used for the Post Source Decay (PSD) measurements on fragments; these measurements are usually difficult or impossible to perform with MALDI.
Porous silicon substrates have become commercially available (WATERS CORPORATION, www.waters.com, and MASS CONSORTIUM, http://www.masscons.com for example).
There are, however, disadvantages to using porous silicon substrates. Porous silicon substrates require special storage and handling conditions. Moreover, even if stored as described by the manufacturer, they become unusable after about a month.
There remains a need for more robust substrates for matrix-free desorption/ionization mass spectrometry.
An object of the present invention is to provide robust porous substrates for matrix-free desorption/ionization mass spectrometry.
Another object of the present invention is to provide porous substrates for desorption/ionization mass spectrometry having a controllable porosity in the mesoporous range of from about 1 nanometer diameter to about 50 nanometers diameter sized pores.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.